1. Field of the Invention
The present invention relates to a sub signal modulation apparatus, a sub signal demodulation apparatus and a sub signal modulation demodulation system which are preferably used in an optical fiber communication system in which a sub signal is superimposed on a main signal and is sent and received.
2. Description of the Related Art
In a conventional communication system using optical fibers for transmission paths, additive information such as monitoring information over transmission paths has been regarded as a sub signal with respect to a main signal serving as a data signal and is sent and received along with the main signal.
Further, transceivers conforming to MSA (Multi Source Agreement) in which a semiconductor laser or an external modulator is included in an optical interface comes to be frequently used as an optical communication apparatus for an optical fiber communication. A normal MSA transceiver is not equipped with a function for sending and receiving a sub signal and therefore requires addition of such a function to the optical communication apparatus in order to realize sub signal communication.
A technique disclosed in patent reference 1 (see the list below) superimposes a sub signal on a main signal which sub and main signals are electric signals and uses the electric signal obtained by the superimposition to control a drive current in a semiconductor laser or an external modulator, so that a light signal in which the sub signal and the main signal are modulated is output.
A technique disclosed in patent reference 2 (also see the list below) incorporates a wave ID insertion unit into a 2R transceiver which insertion unit includes a wave ID modulator for modulating auxiliary data and an intensity modulator for performing an intensity modulation on a light data signal based on the auxiliary data modulated in the wave ID modulator.
In addition to patent references 1, 2, the following patent references 3-6 also disclose techniques having relation to the present invention.
[patent reference 1] Kokai (Unexamined Patent Publication) No. 2002-111596
[patent reference 2] Kokai (Unexamined Patent Publication) No. 2000-165357
[patent reference 3] Kokai (Unexamined Patent Publication) No. HEI 5-267766
[patent reference 4] Kohyo (Unexamined Patent Publication) No. 2003-510960
[patent reference 5] Kokai (Unexamined Patent Publication) No. 2000-134154
[patent reference 6] Kokai (Unexamined Patent Publication) No. HEI 8-186559
Nevertheless, the technique of the patent reference 1 requires a circuit configuration to superimpose a sub signal and a main signal, both electric signals, in the transceiver, and there is a need for a transceiver to which a particular function for subsignal modulation is added. For this reason, it is difficult to apply the technique of the patent reference 1 to a normal MSA transceiver without a sub signal modulation function.
Since the patent reference 2 incorporates the wave ID insertion unit, which modulates a sub signal, in the transceiver, the patent reference 2 has the same problems as the patent reference 1. Besides, partly since of the patent reference 2 does not have a configuration for stabilizing the amplitude of a sub signal and partly since, if the level of a light data signal to be receive varies, a relative level of the amplitude of the sub signal to an intensity of the light data signal also varies, it is difficult to stably transmit the sub signal.
The patent references 3-6 do not disclose techniques able to solve the above problem that a sub signal cannot be stably transmitted which problem is caused by a variation in relative level of the amplitude of the sub signal to the intensity of main signal light serving as light data signal.